40AR* DIFFUSION IN FE-RICH BIOTITE

Abstract

Hydrothermal bulk-loss experiments employing radiogenic Ar (4°Ar*) were performed to determine whether 40 Ar* diffusivity in biotite increases with Fe content. Diffusion laws determined for intermediate and Fe-rich biotite assuming single-domain diffusion (infi- nite-cylinder geometry) are remarkably similar: Fe-mica biotite (Xannite= 0.71) D = 0.40~8:~~exp(-(50500 :t 2.2)/RT) and Cooma biotite (Xannite = 0.54) D = 0.075~8:81rexp (-(47100:t 1.5)/RT). The nearly identical results for Fe-mica biotite and Cooma biotite and their similarity to those from previous studies indicate that most biotite grains of intermediate composition possess comparable 40 Ar* diffusion properties. Because limited grain breakage and volumetrically minor recrystallization is unavoidable during hydro- thermal heating in bulk diffusion experiments, these diffusion laws necessarily provide upper limits to 40 Ar* loss by intercrystalline diffusion. The measured rates of 40 Ar* loss from biotite agree reasonably well with expectations based on single-domain volume dif- fusion using infinite-cylinder geometry when experimental uncertainties are taken into account. However, lack of information regarding 40 Ar* gradients within the hydrother- mally treated mica prevents us from precluding more complex diffusion mechanisms in- volving high diffusivity pathways. In this paper we consider the significance of bulk-loss 40 Ar* diffusion experiments and discuss how diffusion parameters determined in the lab- oratory may be applied to thermochronology provided suitable constraints are available.